'I couldn't swim any further, so I drowned.'
Is the above sentence acceptable?
Point number one: Further describes a matter of degree. "If you have any further interest in this matter, please contact me." Farther describes a comparison of distance. "You live farther from the library than I do."
Point number two: If you had drowned, you'd be dead now.
Q: If a person has drowned, can he speak?
A: Yes, if a dead person can speak.
Even so, most dictionaries seem to give "farther" as one of the definitions of "further". (Cambridge, Macquarie, Oxford, Merriam-Webster)
Another point is that many people believe that spirits can communicate from the other side, and to them, this would be quite a normal sentence.
So, while I personally don't believe this happens, the language still has a need to cater for the possibility for the possibility.